1. Field of the Invention
The present invention relates to image forming apparatuses that form an image using multiple light emitting elements.
2. Description of the Related Art
LED (light emitting diode) systems are known as exposure systems that are employed in electrophotographic image forming apparatuses. In LED systems, an LED array is arranged in the lengthwise direction of a photosensitive drum. Furthermore, a rod lens array may also be provided that condenses the light outputted by the LED array onto the photosensitive drum.
With LED systems, sometimes the relative positional relationship between the LED array and the rod lens array will displace from the ideal relationship, or the relative positional relationship between the rod lens array and the surface of the photosensitive drum will displace from the ideal relationship. In this case, the spot produced on the photosensitive drum will expand undesirably from the ideal size. When the spot expands, it may undesirably overlap with a spot for forming another dot that is adjacent in the main scanning direction or sub scanning direction. This is referred to as spot interference. When spot interference occurs, density change (density unevenness) occurs in the halftones at the areas of interference. In particular, the amount of expansion of the spot related to the displacement in the positional relationship between the lens and the photosensitive drum tends to be greater in LED systems compared to laser beam scanning systems. This is because the distance from the lens to the photosensitive drum in LED systems is much shorter compared to this distance in laser beam scanning systems. Accordingly, mechanical adjustment mechanisms are employed in LED systems to adjust the interval between the LED head and the photosensitive drum.
On the other hand, Japanese Patent Laid Open No. 2002-055498 describes an invention in which an image sample is outputted and an imaging state of the spot is detected from a result of reading the outputted image sample such that density corrections are carried out using image processing in response to the state of imaging. With the invention in Japanese Patent Laid-Open No. 2002-055498, a mechanical adjustment mechanism becomes unnecessary, and therefore halftone density fluctuations caused by expansion of the spot size can be mitigated at low cost.
In this way, the invention described in Japanese Patent Laid-Open No. 2002-055498 is extremely outstanding in regard to the point of being capable of mitigating at low cost the halftone density fluctuations caused by expansion of the spot size. However, there is room for improvement in the invention described in Japanese Patent Laid-Open No. 2002-055498. This is not only because it necessitates a reading detection unit for the sample image, but also because image-forming position misidentification can occur due to error in the reading detection unit.